Does not work well with Markdown formatting

[andrewvaughan]

Markdown is almost impossible to check with proselint because of (at least) the following two checks:

• typography.symbols.sentence_spacing (which fails any time you have a table formatted by prettier, which is another very common linter for Markdown users)
• leonard.exclamation.30ppm (as exclamation points are a very common character for markdown files, as they are part of the language standard - e.g., for images)

There may be more, these are what I've run into thus far.